With current perpendicular magnetic recording technology, the magnetic recording areal density of magnetic recording media (e.g., hard disc platters) has been increased significantly and approaches the physical upper limit due to the superparamagnetic effect. Although magnetic materials with higher coercivity such as FePt, CoPd, etc. have been developed, these materials generally have poor writability due to availability of writing field as a result of saturation of the writing head. Energy assisted magnetic recording (EAMR) or heat assisted magnetic recording (HAMR) technology has been developed as a solution to circumvent the writeability problem and further increase the data areal density to 1 Tbit/in2 and beyond.
In an EAMR system, a near-field transducer (NFT) directs a concentrated field into an adjacent recording medium. Once the medium absorbs the energy of the field, the temperature of the medium increases at a preselected local spot for recording. A waveguide couples light of a preselected wavelength from a light source and focuses the light near the NFT such that the NFT can strongly couple the light energy from the waveguide and form a local surface plasmon. The NFT is a strong absorber of light at resonant status assisted by the surface plasmon effect, and is capable of concentrating the light energy to a very small area (e.g., 40 nm). Therefore, the NFT functions as a relay to deliver the concentrated energy to a recording layer of the medium which can be located several nanometers away from the NFT within the near-field zone.
When the recording medium is heated up, it becomes magnetically soft to allow a writing field to flip the bit data in this small heated area to the desired bit. Due to the resonant coupling of light energy from the waveguide to the NFT, the temperature of the NFT can become very high, even close to its melting point in the worst case. Even if the temperature of the NFT is kept below the melting point, elastic expansion or thermal expansion can cause a protrusion of the NFT (e.g., from an NFT pin or an NFT disk) to the air bearing surface (ABS), which is covered by a thin layer of diamond-like carbon (DLC). Therefore, proper handling of the thermal expansion of the NFT can increase the amount of light power that can be utilized and the lifetime of the EAMR system.